1. Field of the Invention
The present invention relates to a proportional solenoid valve in which a valve element is displaced by applying a current to a solenoid to obtain an output pressure which is proportional to the value of the applied current, and a method for controlling the proportional solenoid valve.
2. Description of the Related Art
In a conventional proportional solenoid valve, a valve element is displaced in accordance with the value of a current applied to a coil while being guided by a cylindrical valve guide portion and is brought into and out of contact with a seat portion, thereby obtaining, from an output port, an output pressure proportional to the value of the applied current (see JP 2002-525524 A, for instance).
In a conventional proportional solenoid valve having this construction, self-induced vibration of the valve element is caused by operating temperature and disturbances such as pulsation. In particular, in recent years, due to rising oil temperatures and the like resulting from size reductions in proportional solenoid valve itself and transmissions, and arrangements in tight spaces, there occurs a situation where there is no choice but to perform control up to the temperature range in which the motion of the valve element becomes unstable. That is, when the oil temperature is high (120 degrees centigrade or higher, for instance) and the valve element is close to the seat portion, there is a danger of the self-induced vibration of the valve element occurring. When the self-induced vibration occurs in this manner, a control pressure is oscillated so that control becomes impossible. There is also a danger that the seat portion may be abraded due to friction between the valve element and the seat portion.